Doktor Kosmos is right.... Disk Utility is the app, found in Utilities in your Applications folder.
You can take any of your burned DVD disks and create a disk image file. It will be smaller in GB size than your iMovie or iDVD projects used to create the DVD disk. Each disk image file will be less than 4.7 GB (for single-layer DVD). You can save disk image files for future burns.
In the future when you create an iDVD project, create a disk image file of the project so you won't have to do it from the burned DVD disk. When you are completely finished with your iDVD Project, do a "Save as Disk Image" from within the project. You will end up with a disk image file. You can use it to burn your DVD disks, using Disk Utility. You can now safely delete the iMovie project and iDVD project and still retain the ability to burn more DVDs of that project from the disk image file.
I have saved ALL of my iDVD projects as disk image files. I did have to put them on a 1T external drive because I have so many (at least 150) but it is comforting to know that I can reburn any of them at any time.
Also, I think Apple's official position is that you cannot create a disk image file of a dual-layer DVD project, but I know many users have done so. If you want more info about that, do a search in the iDVD forum.